NMR Study of Cation Dynamics in Three Crystalline States of 1-Butyl-3-methylimidazolium Hexafluorophosphate Exhibiting Crystal Polymorphism

Abstract

We investigate the cation rotational dynamics of a room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim]PF(6)) in its three crystalline states by (1)H NMR spectroscopy. Spin-lattice and spin-spin relaxation time (T(1) and T(2), respectively) measurements as a function of temperature confirm the presence of three polymorphic crystals of [C(4)mim]PF(6): crystals α, β, and γ, which we previously discovered using Raman spectroscopy and calorimetry. Second moment calculations of (1)H NMR spectra reveal that certain segmental motions of the butyl group in addition to the rapid rotation of the two methyl groups in the cation occur in all the crystals. The trend in the mobility of the segmental motions is γ < β ≤ α, which is consistent with the strength of cation-anion interactions (or crystal packing density) estimated from high-frequency Raman scattering experiments. T(1) measurements demonstrate two types of rotational motions on the nanosecond time scale in all three crystals: fast and slow motions. The three crystals have similar activation energies of 12.5-15.1 kJ mol(-1) for the fast motion, which is assigned to the rotation of the methyl group at the terminal of the butyl group. These observed activation energies were consistent with that estimated by quantum chemical calculations in the gas phase (11.9 kJ mol(-1)). In contrast, the slow motions of crystals α and γ are attributed to different segmental motions of the butyl group and that of crystal β to either a little segmental motion or a certain PF(6)(-) rotational motion. These nanosecond rotational motions obtained from the T(1) measurements do not appear to be affected by crystal packing density because local interactions in the crystalline state rather than packing density govern such nanosecond motions. With respect to the segmental motions, the mobility is likely to change significantly with the conformation of the butyl group. On the basis of these findings, crystal γ, which is the only crystalline phase previously determined using single-crystal X-ray diffraction, is considered to be the most stable phase because of the slowest segmental motions and the strongest cation-anion interactions.

A Comparative Study of the Rotational Dynamics of PF6– Anions in the Crystals and Liquid States of 1-Butyl-3-methylimidazolium Hexafluorophosphate: Results from 31P NMR Spectroscopy

“…However, based on the findings in other RTILs that show crystal polymorphism, 50,51 the differences in the contributions from 31 P− 1 H dipolar interaction between different crystalline polymorphs should be on the order of 10% or less. 31 It should be noted here that the C−H distances in the cations in the crystal structure were obtained from DFT, since these C−H distances obtained from X-ray diffraction are generally underestimated.…”

“…30 Recent studies have shown that this RTIL possesses complex thermal phase behavior and has three polymorphic crystals with the different butyl group conformations, namely, gauche−trans, trans−trans, and gauche′-trans. 30,31 The role of the PF 6 − dynamics may be critical in understanding this unique phase behavior. In this study, we use 31 P nuclear magnetic resonance (NMR) spectroscopic measurements to elucidate the nature of these dynamical processes in [C 4 mim]PF 6 in its crystalline and liquid states.…”

A review of NMR methods used in the study of the structure and dynamics of ionic liquids

“…This indicates that the motion at the root of the butyl group as in [BMMIM][Br] has not been significantly reduced by the C2 substitution in the imidazolium ring . Another report from the same research group confirms the presence of three different polymorphic crystal structures for [BMIM][PF 6 ] named as α , β , and γ ‐crystals studied through the NMR T 1 and T 2 relaxation time . Their relaxation study shows the presence of two types of rotational motions that is the fast and slow motions that are in the time scale of the order of ns range .…”

Thermophysical Properties of Binary Amide Anion-Based Ionic Liquids; TMPAFSA<i>_x</i>TFSA₁₋<i>_x</i>

“…Such melting behavior is particularly common in materials which exhibit polymorphism, and it has been recently reported that ionic liquid exhibits polymorphic behavior. [32][33][34][35][36][37][38][39] Also in this work, there are several solid phases which have different segmental motion due to polymorphism, such solid phase exhibited melting behavior below the temperature corresponding to the solidus line. Additionally, we got composition dependence of heat of fusion as Fig.…”